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1514 Executive summary
This chapter assesses the scientific literature on projected changes in major climate phenomena and more specifically their relevance for future change in regional climates, contingent on global mean temperatures continue to rise. Regional climates are the complex result of processes that vary strongly with location and so respond differently to changes in global-scale influences. The following large-scale climate phenomena are increasingly well simulated by climate models and so provide a scientific basis for understanding and developing credibility in future regional climate change. A phenomenon is considered relevant to regional climate change if there is confidence that it has influence on the regional climate and there is confidence that the phenomenon will change, particularly under the Representative Concentration Pathway 4.5 (RCP4.5) or higher end scenarios. {Table 14.3} Monsoon Systems There is growing evidence of improved skill of climate models in reproducing climatological features of the global monsoon. Taken together with identified model agreement on future changes, the global monsoon, aggregated over all monsoon systems, is likelyIn this Report, the following terms have been used to indicate the assessed likelihood of an outcome or a result: Virtually certain 99–100% probability, Very likely 90–100%, Likely 66–100%, About as likely as not 33–66%, Unlikely 0–33%, Very unlikely 0–10%, Exceptionally unlikely 0–1%. Additional terms (Extremely likely: 95–100%, More likely than not >50–100%, and Extremely unlikely 0–5%) may also be used when appropriate. Assessed likelihood is typeset in italics, e.g., very likely (see Section 1.4 and Box TS.1 for more details). to strengthen in the 21st century with increases in its area and intensity, while the monsoon circulation weakens. Monsoon onset dates are likely to become earlier or not to change much and monsoon retreat dates are likely to delay, resulting in lengthening of the monsoon season in many regions. {14.2.1} Future increase in precipitation extremes related to the monsoon is very likely in South America, Africa, East Asia, South Asia, Southeast Asia and Australia. Lesser model agreement results in medium confidenceIn this Report, the following summary terms are used to describe the available evidence: limited, medium, or robust; and for the degree of agreement: low, medium, or high. A level of confidence is expressed using five qualifiers: very low, low, medium, high, and very high, and typeset in italics, e.g., medium confidence. For a given evidence and agreement statement, different confidence levels can be assigned, but increasing levels of evidence and degrees of agreement are correlated with increasing confidence (see Section 1.4 and Box TS.1 for more details). that monsoon-related interannual precipitation variability will increase in the future. {14.2.1, 14.8.5, 14.8.7, 14.8.9, 14.8.11, 14.8.12, 14.8.13} Model skill in representing regional monsoons is lower compared to the global monsoon and varies across different monsoon systems. There is medium confidence that overall precipitation associated with the Asian-Australian monsoon will increase but with a north–south asymmetry: Indian and East Asian monsoon precipitation is projected to increase, while projected changes in Australian summer monsoon precipitation are small. There is medium confidence that the Indian summer monsoon circulation will weaken, but this is compensated by increased atmospheric moisture content, leading to more precipitation. For the East Asian summer monsoon, both monsoon circulation and precipitation are projected to increase. There is medium confidence that the increase of the Indian summer monsoon rainfall and its extremes throughout the 21st century will be the largest among all monsoons. {14.2.2, 14.8.9, 14.8.11, 14.8.13} There is low confidence in projections of changes in precipitation amounts for the North American and South American monsoons, but medium confidence that the North American monsoon will arrive and persist later in the annual cycle, and high confidence in expansion of the South American monsoon area. {14.2.3, 14.8.3, 14.8.4, 14.8.5} There is low confidence in projections of a small delay in the development of the West African rainy season and an intensification of late-season rains. Model limitations in representing central features of the West African monsoon result in low confidence in future projections. {14.2.4, 14.8.7} Tropical Phenomena Based on models’ ability to reproduce general features of the Indian Ocean Dipole and agreement on future projections, the tropical Indian Ocean is likely to feature a zonal (east–west) pattern of change in the future with reduced warming and decreased precipitation in the east, and increased warming and increased precipitation in the west, directly influencing East Africa and Southeast Asia precipitation. {14.3, 14.8.7, 14.8.12} A newly identified robust feature in model simulations of tropical precipitation over oceans gives medium confidence that annual precipitation change follows a ‘warmer-get-wetter’ pattern, increasing where warming of sea surface temperature exceeds the tropical mean and vice versa. There is medium confidence in projections showing an increase in seasonal mean precipitation on the equatorial flank of the Inter-Tropical Convergence Zone (ITCZ) affecting parts of Central America, the Caribbean, South America, Africa and West Asia despite shortcomings in many models in simulating the ITCZ. There is medium confidence that the frequency of zonally oriented South Pacific Convergence Zone events will increase, with the South Pacific Convergence Zone (SPCZ) lying well to the northeast of its average position, a feature commonly reproduced in models that simulate the SPCZ realistically, resulting in reduced precipitation over many South Pacific island nations. Similarly there is medium confidence that the South Atlantic Convergence Zone will shift southwards, leading to an increase in precipitation over southeastern South America and a reduction immediately north thereof. {14.3, 14.8.4, 14.8.5, 14.8.7, 14.8.11, 14.8.14} There is low confidence in projections of future changes in the Madden–Julian Oscillation owing to poor ability of the models to simulate it and its sensitivity to ocean warming patterns. The implications for future projections of regional climate extremes in West Asia, South Asia, Southeast Asia and Australia are therefore highly uncertain when associated with the Madden–Julian Oscillation. {14.3, 14.8.10, 14.8.11, 14.8.12, 14.8.13} There is low confidence in the projections of future changes for the tropical Atlantic, both for the mean and interannual modes, because of systematic errors in model simulations of current climate. The implications for future changes in Atlantic hurricanes and tropical South American and West African precipitation are therefore uncertain. {14.3, 14.6.1, 14.8.5, 14.8.7 } The realism of the representation of El Niño-Southern Oscillation (ENSO) in climate models is increasing and models simulate ongoing ENSO variability in the future. Therefore there is high confidence that ENSO very likely remains as the dominant mode of interannual variability in the future and due to increased moisture availability, the associated precipitation variability on regional scales likely intensifies. An eastward shift in the patterns of temperature and precipitation variations in the North Pacific and North America related to El Niño and La Niña (teleconnections), a feature consistently simulated by models, is projected for the future, but with medium confidence, while other regional implications including those in Central and South America, the Caribbean, Africa, most of Asia, Australia and most Pacific Islands are more uncertain. However, natural modulations of the variance and spatial pattern of ENSO are so large in models that confidence in any specific projected change in its variability in the 21st century remains low. {14.4, 14.8.3, 14.8.4, 14.8.5, 14.8.7, 14.8.9, 14.8.11, 14.8.12, 14.8.13, 14.8.14} Cyclones Based on process understanding and agreement in 21st century projections, it is likely that the global frequency of occurrence of tropical cyclones will either decrease or remain essentially unchanged, concurrent with a likely increase in both global mean tropical cyclone maximum wind speed and precipitation rates. The future influence of climate change on tropical cyclones is likely to vary by region, but the specific characteristics of the changes are not yet well quantified and there is low confidence in region-specific projections of frequency and intensity. However, better process understanding and model agreement in specific regions provide medium confidence that precipitation will be more extreme near the centres of tropical cyclones making landfall in North and Central America; East Africa; West, East, South and Southeast Asia as well as in Australia and many Pacific islands. Improvements in model resolution and downscaling techniques increase confidence in projections of intense storms, and the frequency of the most intense storms will more likely than not increase substantially in some basins. {14.6, 14.8.3, 14.8.4, 14.8.7, 14.8.9, 14.8.10, 14.8.11, 14.8.12, 14.8.13, 14.8.14} Despite systematic biases in simulating storm tracks, most models and studies are in agreement on the future changes in the number of extratropical cyclones (ETCs). The global number of ETCs is unlikely to decrease by more than a few percent. A small poleward shift is likely in the Southern Hemisphere (SH) storm track. It is more likely than not, based on projections with medium confidence, that the North Pacific storm track will shift poleward. However, it is unlikely that the response of the North Atlantic storm track is a simple poleward shift. There is low confidence in the magnitude of regional storm track changes, and the impact of such changes on regional surface climate. It is very likely that increases in Arctic, Northern European, North American and SH winter precipitation by the end of the 21st century (2081–2100) will result from more precipitation in ETCs associated with enhanced extremes of storm-related precipitation. {14.6, 14.8.2, 14.8.3, 14.8.5, 14.8.6, 14.8.13, 14.8.15} Blocking Increased ability in simulating blocking in models and higher agreement on projections indicate that there is medium confidence that the frequency of Northern and Southern Hemisphere blocking will not increase, while trends in blocking intensity and persistence remain uncertain. The implications for blocking-related regional changes in North America, Europe and Mediterranean and Central and North Asia are therefore also uncertain. {14.8.3, 14.8.6, 14.8.8, Box 14.2} Annular and Dipolar Modes of Variability Models are generally able to simulate gross features of annular and dipolar modes. Model agreement in projections indicates that future boreal wintertime North Atlantic Oscillation is very likely to exhibit large natural variations and trend of similar magnitude to that observed in the past and is likely to become slightly more positive on average, with some, but not well documented, implications for winter conditions in the Arctic, North America and Eurasia. The austral summer/autumn positive trend in Southern Annular Mode is likely to weaken considerably as stratospheric ozone recovers through the mid-21st century with some, but not well documented, implications for South America, Africa, Australia, New Zealand and Antarctica. {14.5.1, 14.5.2, 14.8.2, 14.8.3, 14.8.5, 14.8.6, 14.8.7, 14.8.8, 14.8.13, 14.8.15} Atlantic Multi-decadal Oscillation Multiple lines of evidence from paleo reconstructions and model simulations indicate that the Atlantic Multi-decadal Oscillation (AMO) is unlikely to change its behaviour in the future as the mean climate changes. However, natural fluctuations in the AMO over the coming few decades are likely to influence regional climates at least as strongly as will human-induced changes, with implications for Atlantic major hurricane frequency, the West African wet season, North American and European summer conditions. {14.7.6, 14.2.4, 14.6.1, 14.8.3, 14.8.6} Pacific South American Pattern Understanding of underlying physical mechanisms and the projected sea surface temperatures in the equatorial Indo-Pacific regions gives medium confidence that future changes in the mean atmospheric circulation for austral summer will project on this pattern, thereby influencing the South American Convergence Zone and precipitation over southeastern South America. {14.7.2, 14.8.5}